The invention is in the field of solar cells and is particularly directed to protecting the solar cell p-n junction when interconnections are made between solar cells to form an array of solar cells.
A solar cell is another name for a photovoltaic cell, the latter being a device which responds to light causing a potential difference between two terminals of said device. The most common solar cell in use today is a silicon cell, but other semiconductor materials are also known to be suitable for solar cells.
The basic parts of a cell are a semiconductor material having a p-n junction therein below the upper surface of the cell, a back electrode, and a front electrode. The front electrode is always constructed to allow light to pass into the cell through the upper surface thereof. Typically, the front or top electrode comprises a number of thin metallic fingers connected to a larger metallic land portion. The land portion, as defined herein, is the portion of the top electrode to which an external interconnector is attached. External interconnection is necessary to form complete solar cell arrays.
Most solar cells also have anti-reflective coatings which cover at least so much of the top surface which is not covered by the top electrode. The anti-reflective coating serves the purpose of preventing useful radiation from being reflected off the surface of the cell. The U.S. Pat. No. 3,533,850 to Tarneja et al teaches a number of anti-reflective coatings and associated methods of fabricating cells with anti-reflective coating. The following U.S. patent applications, assigned to the assignee herein, teach particular anti-reflective coatings and methods of fabricating solar cells with the anti-reflective coatings thereon:
I. "tantalum Pentoxide Anti-Reflective Coating," by Lindmayer et al, Ser. No. 249,024, filed May 1, 1972. PA1 Ii. "niobium Pentoxide Anti-Reflective Coating," by Lindmayer et al, Ser. No. 331,741, filed Feb. 13, 1973. PA1 Iii. "method of Applying Anti-Reflective Coating to a Solar Cell," by Lindmayer et al, Ser. No. 331,739, filed Feb. 13, 1973.
Most present solar cells also include a quartz cover slide placed over the anti-reflective coating and top electrode. The quartz cover slide protects the cell from harmful radiation.
In the past most solar cell arrays used in extra-terrestrial operation, such as on earth satellites, were body mounted on the space vehicle. Although the body mounted arrays passed into and out of the sun's rays, the body of the space vehicle prevented the thermal cycling of the arrays from being so great as to cause failure or fatigue of the solder which was typically used to connect the interconnectors to the lands. More recently, solar arrays have been sun oriented. That is, rather than being directly mounted on the vehicle body they have been mounted on wings or flaps which extend away from the vehicle body and which orient the solar arrays toward the sun. While sun oriented arrays have obvious benefits, one of the disadvantages of such arrays is that solder is no longer suitable to connect the interconnectors to the lands. The thermal cycling of sun oriented arrays is so great that the solder cracks.
As a result of the above problem, solder has been eliminated, and bonding methods such as welding and thermal compression bonding are used to secure the interconnectors to the lands. The problem with these methods of bonding is that they introduce mechanical pressure and large amounts of heat to the cell at the land. This tends to destroy the p-n junction locally below the land, resulting in an I-V curve for the cells which is not as sharp as it otherwise would be. Ultrasonic bonding may also be used but it creates the same problem as welding and thermal compression bonding.
The problem is even more acute for a particular preferred high-efficiency solar cell of the type described in U.S. patent application, "Fine Geometry Solar Cell," by Lindmayer, Ser. No. 184,393, filed Sept. 28, 1971 and assigned to the assignee herein. In the fine geometry solar cell the improved electrode pattern allows the p-n junction to be placed much closer to the surface than in prior art cells, with the result that overall efficiency is increased. However, these cells, due to the closeness of the p-n junction to the surface, are even more susceptible to local destruction of the p-n junction below the land during bonding.